An aircraft tail assembly is typically formed from a horizontal tail and another vertical tail. The horizontal tail is in charge of the pitch control and stability of the aircraft, whereas the vertical tail is in charge of the lateral control and stability of such aircraft. The planform of a tail assembly is the contour of the projection on its plane, the aerodynamic properties thus depending to a great extent on this planform. The tail assembly rudder is the moving part thereof which allows controlling the aircraft.
The tail assembly span is the size of the tail assembly in a direction perpendicular to the impinging current, a tail assembly section being considered as a tail assembly slice in the direction of the impinging air current. The chord in a specific section is therefore the size of the tail assembly in that section, the chord at the root and the chord at the tip being the chords at the root and at the tip of the tail assembly, respectively. The chord distribution is the mathematical function expressing the chord according to the position along the span. This function is normally a decreasing function, being constant for the case of rectangular tail assemblies.
The aerodynamic lift is the aerodynamic force supported by the tail assembly in the direction perpendicular to the impinging air current. The aerodynamic drag is the aerodynamic force supported by the tail assembly in the direction of the impinging current.
Some important factors to be taken into account in the first stages of studying an airplane are: the area enclosed by the planform or total surface; the distance from the tail assembly to the center of gravity of the airplane and the surface of the rudder with respect to the total surface. Once these parameters are set, the design consists of optimizing the tail assembly, minimizing its weight, its aerodynamic drag, its manufacturing costs and improving the efficiency of the rudder.
There are several possible tail assembly planforms for aircraft. The simplest planforms are rectangular or trapezoidal. Wings (not tail assemblies) were historically manufactured with elliptical planforms, because the aerodynamic wing theory predicts that this planform minimizes aerodynamic drag. It is currently known that an elliptical planform is more expensive to manufacture and heavier than a trapezoidal planform and that the aerodynamic drag loss is small.
Rectangular and trapezoidal planforms of tail assemblies are the most widely used due to the fact that they have a great structural rigidity and simplicity, while at the same time they are planforms which can easily be analyzed by computer and can be manufactured inexpensively. However, for high-performance applications they do not provide a multidisciplinary optimum if weight, aerodynamic drag and manufacturing costs are taken into account.
The internal structure of an aircraft tail assembly traditionally comprises two main spars, a front spar and a rear spar, distributed along the span, closing the aircraft tail assembly structure or box. The position of the front and rear spars in each section is a constant percentage of the chord in each section. A typical percentage of the position of the front spar would therefore be 20% of the chord in each section, whereas a typical percentage of the position of the rear spar would be 55% of the chord in each section. There are tail assemblies with three or more spars but their position is always a constant percentage of the chord in each section.
In a similar manner, the hinge line of the rudder (elevator or rudder) has a constant percentage along the span, typically 70% of the chord in each section. Due to its function as an axis about which the rudder rotates, the hinge line must be rectilinear, although it is not necessary for it to have a constant percentage of the chord in each section, this being the object of the present patent.
The multidisciplinary optimization of an aircraft tail assembly (hereinafter multidisciplinary optimization) consists of modifying its planform, the size of the rudder, the position of its hinge line and the position of its spars such that its weight, its aerodynamic drag, the efficiency of its rudder and its manufacturing costs are simultaneously optimized.
The object of the present patent is to provide innovative planforms, rudder shapes, position of the hinge line and position of the spars, based on a multidisciplinary optimization and for an aircraft tail assembly.